Object recognition and detection is important in a great variety of automated systems. Apparatus and methods for determining the heights of objects disposed upon a background field or for recognizing and locating objects of a particular, preselected height, within a visual field, are of great utility in a variety of applications.
Height is one readily knowable, and relatively invariant physical constant of an object and can provide the basis for the detection of the presence of particular objects. Remote, i.e., non-contact, sensor systems which are capable of detecting the heights of objects, or alternatively, identifying objects of a given height, have applications in many areas. For example, systems of this type may be readily adapted to inspect fabricated articles for dimensional tolerances. Similarly, materials handling systems may be provided with height recognition capabilities to allow them to identify and classify individual members of a mixed assembly of parts and appropriately direct each of the items to an appropriate destination.
Height detection capability would be of great utility in a target acquisition and/or identification system since the height of a variety of objects of military significance such as tanks, trucks, armored personnel carriers and other such vehicles is a readily knowable and invariant feature. A sensing system with height detection capability will allow for a ready selection of significant targets, from a plurality of potential targets in a zone. Similarly, height detection allows for good correlation of an identified target with its origin, i.e., friend or foe. A sensor having the ability to detect heights of objects can be utilized to identify and classify the nature of potential targets as well as direct weapons and/or tracking systems to the target.
The earliest measurements of object heights were implemented with techniques which rely upon the use of standards and require contact with the object being measured. More refined non-contact techniques soon evolved which employ trigonometry or other mathematical analyses to deduce object height. Relatively recently, non-contact optical techniques have been implemented for height measurement. One prior art optical technique involves the use of a pair of stereo images, that is to say, a pair of photographic images made from two different points of view. If the distance to the object and the spacing of the two points of view is known, the images may be processed to determine the height of various features therein. This technique requires creation of a pair of images as well as a relatively complex processing operation.
Interferometric techniques are quite well known for the measurement of distances, as well as heights. Such techniques rely upon the wave aspects of a beam of electromagnetic energy. One type of interferometric technique involves the heterodyning of two light beams of differing frequencies; and in one general embodiment, a pair of light beams of differing frequencies are directed onto an object and onto a reference plane. The return beams are processed in various manners to determine the distance and/or height of objects, based upon the patterns of the reflected waves. Such techniques are disclosed in U.S. Pat. Nos. 4,650,330; 4,744,653 and 4,572,669. Techniques such as the foregoing generally utilize rather complicated optical systems to provide, direct and sense the multiple beams of different frequencies. In general, techniques of this type provide for very accurate height and distance measurement; however, the useful range of operations is typically of the magnitude of the light waves themselves. That is to say, such systems are generally utilized for measurements in the sub-micron to micron size range. As a consequence, systems such as those disclosed in the above-referenced patents are not utilized for the measurement of the heights of large objects.
U.S. Pat. No. 4,689,489 shows a technique wherein height analysis is implemented in a macroscopic system. Disclosed therein is a method for measuring the height of fluid in tanks which involves modulating a beam of visible light at radio frequencies, reflecting the beam from the fluid in the tank and comparing the reflected beam with a reference beam. The frequency of modulation is adjusted until the reflected beam and reference beam are matched and the ratio of frequencies is utilized to determine fluid height. A system of this type requires a separate reference beam and is relatively slow in operation; accordingly, it is not suited for situations where rapid data acquisition is needed. Systems of this type are not suitable for the aforementioned military applications.
Accordingly, it will be appreciated that there is needed a simple, quick and low-cost method and apparatus for remotely determining the height of relatively large objects. The present invention meets the foregoing criteria. As will be disclosed in greater detail hereinbelow, the present invention provides a sensor which relies upon interference phenomenon in a single beam of electromagnetic energy modulated at appropriate frequencies. It is to be understood that the methods and apparatus of the present invention may be implemented with electromagnetic energy of any frequency. Consequently, as utilized herein, the term "light" shall refer to any portion of the electromagnetic spectrum and the term "optics" shall not be limited to any particular portion of the electromagnetic spectrum.
The phenomenon of interference occasioned by reflection of a monochromatic beam of electromagnetic energy from a quarter-wave plate is well-known in the optical arts. Briefly put, a beam of monochromatic energy reflected from the two portions of a step dimensioned to be one-fourth wavelength of the monochromatic beam (or odd multiples thereof) undergoes destructive interference. That is to say, the energy is equally reflected from the top and bottom of the step and interferes to produce a reflected signal of theoretically zero intensity. Conversely, if the step is dimensioned to be one-half wavelength (or integer multiples thereof), the reflected energy undergoes constructive interference and the reflected signal is a maximum.
In accord with the principles of the present invention, detection of objects of a preselected height, within a target zone, may be readily accomplished by directing a beam of electromagnetic energy modulated at a frequency "f.sub.1 " such that f.sub.1 =(c/4h), wherein c is the speed of light and h is the preselected height. If objects of the preselected height are within that zone, they will create a quarter-wave step in conjunction with the background field and the signal reflected therefrom will be a null, or low value. The locations of the null signals will correlate with objects of the preselected height. It should be noted that the frequency of modulation can, (and usually does) differ from the basic frequency of the beam.
Referring now to FIG. 1, there is shown a schematic representation of the intensity of a beam of electromagnetic energy, modulated at a given frequency f, as reflected from objects of various heights. The modulation frequency f is selected such that objects of the height "h" will create a quarter-wave situation, (i.e., f=c/4h). As it will be seen, the signal has minima at the points corresponding to the height h, as well as odd multiples thereof. In principle, a system of this type may be readily employed to detect objects of a preselected height "h" as well as to ascertain heights of other objects. It will be noted that a maximum signal is obtained at values half the selected height and multiples thereof, and that signal intensity between maxima and minima will correlate with object height. While under idealized situations, a response such as that of FIG. 1 is provided utilizing the techniques disclosed herein, in reality problems can occur because various objects in a target zone, the background field and even different portions of a single object reflect the beam of energy with different efficiencies.
These problems are illustrated with reference to FIG. 2 where there is shown a representation of the intensity of reflected electromagnetic energy versus object height generally similar to that of FIG. 1 but for four different levels of reflectivity p.sub.1 -p.sub.4. As will be noted, the position of the maxima and minima are relatively unchanged, however the magnitude of the maxima varies. Information such as that represented in FIG. 2 is of little use in height determination. While a simple pattern of data such as that of FIG. 1 would allow one to determine height of an object by measuring the intensity of the reflected electromagnetic energy, obviously such could not be done with the data of FIG. 2. It would not be known whether a given signal level represents maximum for a low reflectivity object or if it were a value tending toward a minimum for a higher reflectivity object. The complex reflection pattern also tends to make detection of minima very difficult.
It will thus be appreciated that there is a need to "normalize" the effects of non-uniform reflectivity so as to provide an output signal indicative of height but independent of variations in reflectivity of objects in a target zone. The present invention provides for a method and apparatus for sensing object height which allows for the normalizing or negating of the effects of non-uniform reflectivity. The device of the present invention is referred to as a "Range Dispersion Sensor" and it is a device which is relatively simple to fabricate and use and which has the capability of rapidly, simply and economically acquiring and/or measuring the height of an object in a target zone with a single sensor scan. The techniques disclosed herein permit very simple decision logic to be implemented for target detection, classification and/or warhead fusing cues. With multiple scans, the range dispersion techniques disclosed herein permit visual imaging of the edges of objects. These and other features of the present invention will be readily apparent from the drawings, discussion, description and claims which follow.